Fluid filtration is a well-known industrial operation. Many different types of filter apparatus have been developed, ranging from units for separation of relatively coarse particles from a fluid stream down to the removal of sub-micron particles. All types of filter media have been used, having varying degrees of effectiveness depending on the type of fluid being processed and the kind of particle being removed. The filtration operation is basically simple, being essentially a straining and size separation procedure. In many cases the rate of filtration is a complicating factor, so that a filter which is effective at a given fluid flow rate may quickly become ineffective when flow rates are increased. Fluid composition may also give rise to undesirable effects. Fluids of high purity may have such resistance to the conductance of electricity that a static charge is built up within the fluid from the action of flowing through piping or similar conduits. Nonconductive filters with their multiplicity of orifices or pores break up the fluids into very small portions, resulting in the accumulation of the static charge on the nonconductive filter media. When this potential is sufficiently high, a spark may jump to a surface of lower potential with the possibility of either igniting the fluid, if a flammable liquid or gas, or causing an explosion if the fluid is a suspension of dust in air, for example.
In some cases a low potential charge on a filter medium may be beneficial, especially if the charge is of opposite polarity to that of the particles being separated. In this case, the electrical attraction of the filter medium for the particles enhances filter action. This type of charge may be supplied from a controlled external source of direct current and requires a filter media with electrical conductance.
A number of patent references describe filters in which the formation of a static charge is relied on to enhance filter efficiency. In U.S. Pat. No. 2,612,966 to W. H. Nicol, a filter medium of polyethylene fibers is disclosed for air filtration, passage of the air building up an electrostatic charge which removes dust particles. The filter medium is nonconductive and the static charges are of uncontrolled potential. Other patents, such as U.S. Pat. No. 2,724,457 to C. S. Besser and U.S. Pat. No. 2,795,290 to P. V. Bertsch, disclosed similar types of filters. A U.S. Pat. No. 2,992,700 to L. Silverman discloses a fluidized bed of plastic spheres which build up static charges during filtration. In U.S. 3,186,551 issued to A. Dornauf, a filtering method for jet fuel is described which indicates a conductive connection between two metal end caps and the center core of the filter cartridge to remove static electricity. This is a primitive method and does not envision the presence of static electricity throughout the fluid which does not conduct away easily. A patent issued to D. Zulauf (U.S. Pat. No. 3,446,906) describes a metal plated plastic foam sheet material which would be more effective for static charge removal, no mention is made of fibers or conductive materials other than metals. In U.S. Pat. No. 2,684,126, issued to D. I. Doyle, the hazard of dust explosion by static electrical discharge is cited and a grounded copper wire screen insert for a filter bag is disclosed.
From a consideration of the prior art, it appears desirable to have some type of electrical charge on the filter media, especially for applications in which very small particles are to be removed from the fluid stream. Static electricity is generated at relatively high potentials and none of the prior art devices seem to have any way of regulating this potential. There is probably little hazard for small filters handling low volumes of fluid at relatively low velocities such as domestic vacuum or furnace air cleaners. In larger scale operations, however, large volumes of fluid are handled at high velocities and the amounts of flammable liquids or explosiveprone particle and gas mixtures present a serious fire or explosion hazard. The control of electrical charges on filter media is important; therefore, an object of the present invention is to provide filter media which have conductive properties which permit removal of dangerous static electrical potentials from fluid streams. A further object of the invention is the provision of filter media which will allow the establishment of a controlled low voltage potential on the media to aid particle separation while avoiding the hazards of a high potential discharge from the filter media into the filter fluid.